Support VCS-friendly plugins rev lockfile

[lttb]

Thanks a lot for such an amazing shell plugin manager!

Feature Suggestion: a trackable versions lockfile would be an amazing addition to the existing lock functionality.

Example

sheldon lock generates $XDG_CONFIG_HOME/sheldon/plugins.lock if it doesn't exist (similarly to $XDG_DATA_HOME/sheldon/plugins.lock), with automatically pinned rev versions.

For example,

[[plugin]]
name = "zsh-defer"
source = "github:romkatv/zsh-defer"
rev = "53a26e287fbbe2dcebb3aa1801546c6de32416fa"

[[plugin]]
name = "evalcache"
source = "github:mroth/evalcache"
rev = "3153dcd77a2c93aa8fdf5d17cece7edb1aa3e040"

This file can be tracked in the VCS along with with plugins.toml and other dotfiles.

• sheldon lock --update updates the $XDG_CONFIG_HOME/sheldon/plugins.lock
• sheldon lock and sheldon lock --reinstall installs dependenies based on the lock file if exists

Notes

• I suppose it can be an optional flag in sheldon config (plugins.toml), disabled by default?
• $XDG_CONFIG_HOME/sheldon/plugins.lock naming is just an example, I guess there can be a better name to avoid confusion with existing sheldon/plugins.lock file
• Plugins can be remote and local too, so it's probably fair to assume that rev locking is available only for github/gist plugins

What do you think?